The present invention relates to the extraction of metals and their salts which are amalgamatable with mercury, in particular gold, and which are present in dispersed form in slurries such as for example alluvial sands. In the following description specific reference will be made to gold and to alluvial sands it being understood that such reference is not intended as a limitation but only as an example. Indeed it is known that with the exception of iron almost all other metals amalgamate to a greater or lesser extent with mercury so that the method and the apparatus in accordance with the invention may be applied in a similar manner.
It is also known that alluvial sands and deposits contain gold in very small proportions, of the order of 1 gram approximately for each ton of river sand and that such gold is found in various physical forms including lamellae, particles occlusions in grains, e.g. quartz, of varying grain size.
It is also known from the literature that gold combines very readily with mercury to form an alloy which takes the name amalgam and from which it can recovered equally readily.
If an intimate contact is made between the sand and the mercury, recovery of gold from the sand can reach 100% of the gold contained in the sand. Finally, it should not be forgotten that alluvial sands are used in the construction industry, for which they are extracted from quarries, manipulated, and handled often in the form of slurries and aqueous solutions.
Recovery of gold from alluvial sand with the amalgam formation method was well known in the rather distant past and is still in use today albeit sporadically, having been replaced by the cyanide process. The amalgam formation method consists of passing a slurry or an aqueous solution of sand over a surface made of copper amalgamated with mercury preferably in the presence of silver.
After a certain period of time, of the order of at least several days, i.e. when the mercury's amalgamation capacity is believed to be exhausted, the copper and mercury amalgam support bearing the gold also amalgamated with the mercury is treated for recovery of the gold and optionally for restoration of the support.
Although the preceding description is extremely concise it can readily be appreciated that the known method presents two serious disadvantages and problems, i.e. it is entirely empirical and allows no control of the process and in the second place but not of secondary importance it has the disadvantage of involving totally manual operations. It is clear that these problems and disadvantages preclude any modern industrial use of this process.
On the other hand even the cyanide process presents drawbacks and risks. Finally it is worth noting that a quarry of not excessive size processes daily quantities of gravel and sand of the order of hundreds of tons so that the real interest which the recovery of the gold in such sand could have, and indeed has, can be quickly understood. The main problem connected with reovery of the gold content of alluvial sands is however the formation of a surface film of mercury sulfides. Indeed often because of polluting substances contained in river water and also because of sulfur and/or sulfides present in alluvial sands, the sulfides combine with the mercury to form an impermeable film on the surface of the mercury which inhibits amalgamation with the gold. The main object of the present invention is to solve said problems and drawbacks.
One specific object of the present invention is to provide a method and equipment which would allow recovery, in an acceptable and profitable manner, of metals which are amalgamatable with mercury, more specifically noble metals and still more specifically silver and gold, from alluvial sands and aqueous slurries.